1. Field of the Invention
The present invention relates to heat exchangers for furnaces. More specifically, the field of the invention is that of heat exchanger tubes which provide passageways for heated flue gases within furnace heat exchangers.
2. Prior Art
Tubular conduits are used in heat exchangers to provide an interior conduit for flue gases and exterior heat transfer surface for circulating air. The interior conduits may be formed from metallic clam-shell plates wherein two clam-shell plate surfaces are connected together to form the conduits, or the tubular conduits may comprise metal tubes. Within furnaces, such tubular conduits provide a passageway for flue gases, the heated products of combustion, which flow through the heat exchanger. The flue gas flow transfers heat to the material defining the passageway which then transfers the heat to air circulating over and around the heat exchanger.
The heat exchanger contains the flue gas flow. An inducer fan draws the combustion gases from a gas burner through the passageway to an exhaust system. The inducer fan insures that the heated flue gases are constantly flowing through the heat exchanger during the operation of the furnace, providing sufficient air for combustion. Also, a circulator fan is disposed adjacent the heat exchanger to drive a flow of circulation air over and around the tubular conduit and into the interior of the building being heated.
Typically, a furnace's heat exchanger tubular conduit contains one or more elongate portions which are disposed perpendicularly to the flow of circulation air. This arrangement allows the circulation air flow to impact on the exterior surfaces of the heat exchanger conduits to promote heat exchange. Also, the conduits generally include two or more elongated sections connected by bend sections so that the interior flow of heated flue gas is disrupted and impacts interior surfaces of the tubes to promote further heat exchange. However, a problem with prior art heat exchanger conduits involves the inefficiency in the amount of heat transferred from the heated flue gases to the circulating air.
Various structures exist which increase heat transfer efficiency. For example, one known configuration includes a plurality of indentations within the wall of the heat exchanger for disrupting the flow over the indentations. A problem with this configuration is that although disruption is caused within the flow along the inner surfaces of the wall, the flow in the center may only be minimally effected. Another known configuration includes tubes which have curved or polygonal walls varying in cross-sectional shape over the length of the tube. A problem with this configuration is the expense involved in manufacturing tubes which vary in cross-sectional shape over their length.
Inlets and outlets of the heat exchanger conduits are attached to a heat exchanger panel so that the burners, inducer and circulator fans, and the exhaust system can be conveniently attached to the heat exchanger. The heat exchanger conduits are disposed within the heat exchanger and arranged so that the circulator fan drives air over the conduits. For the clam-shell configuration, the plates are disposed generally perpendicularly to the direction of circulation air flow. The problem with the clam-shell configuration is that the flow produced by the circulator fan is only minimally disrupted in the spaces between the plates. For the tubular configuration, generally cylindrical elongated portions of the tubes may be disposed so that a direct line of sight is blocked along the direction of circulation flow. A problem with the tubular configuration is that the first row of cylindrical elongated portions causes a high pressure drop in the circulation flow resulting in the circulation flow only minimally wrapping around the other row or rows of elongated portions so that hot spots develop on the downstream elongated portions. Also, a relatively large circulation fan must be used to provide a sufficiently strong flow of circulation air following the high pressure drop.
What is needed is a heat exchanger element which more efficiently transfers heat from the heated flue gases to the circulation air.
Another need is for a heat exchanger element in which the flow in the center of the conduit is more effectively disrupted.
A further need is for such a heat exchanger element which is less expensive to manufacture.
A still further need is for a heat exchanger conduit for a furnace which promotes circulation air flow around the exterior of the conduit and minimizes the occurrence of hot spots.
Also needed is a heat exchanger conduit for a furnace which reduce the pressure drop of the circulation air across the heat exchanger conduits.